1. Field of the Invention
This invention relates to a method for producing a gold fine particle-supported carrier catalyst for a fuel cell, in which a gold fine particle with an average particle diameter of nanometer order is supported. The invention also relates to a polymer electrolyte fuel cell catalyst that contains a gold fine particle, and a polymer electrolyte fuel cell.
2. Description of the Related Art
A fuel cell is required to exhibit high power generation performance over a long period of time; a period of 5000 hours is required in an automotive power supply and a period of 40,000 hours in a fixed power supply. Thus, an electrode catalyst needs to have high catalytic activity and durability. An electrode catalyst that has a porous carbon particle supporting noble metals, base metals, or other catalytic metals is used as the electrode catalyst of high catalytic activity and durability. A method that is generally used as a method for producing the electrode catalyst that has a carbon particle supporting a plurality of noble metals as catalytic metals is, for example, an absorbing method for dispersing and mixing a carbon particle in an aqueous solution that contains a plurality of noble metal compounds, forming a noble metal particle on the carbon particle by adding a reducing agent or precipitant to the mixture, and thereafter burning thus obtained complex noble metal particle.
However, this method causes the plurality of noble metal particles formed by an insolubilization agent to be absorbed randomly by the other noble metal particles or the carrier surface. Therefore, the composition of the alloyed complex noble metal particle formed by burning such noble metal particles becomes uneven and sinters due to the addition of thermal energy, resulting in forming a large particle diameter. As a result, the active area of the complex noble metal particles is reduced, lowering the catalytic activity. In addition, when the absorbing method is used, although the complex noble metal particle is formed/supported on a pore of the carbon particle to which an electrolyte does not sufficiently penetrate, such complex noble metal particle that is not capable of coming into contact with the electrolyte does not sufficiently function as an active component of the electrolyte catalyst. Therefore, the problem was that the effective utilization factor of the supported complex noble metal particle is reduced.
On the other hand, platinum, palladium, or other noble metal is used as a catalyst for a fuel cell catalyst (a catalyst for a fuel cell) or for exhaust purification. However, because the noble metal elements exist on the earth in a limited amount, the usage thereof needs to be reduced as much as possible. Therefore, as a catalyst using a noble metal, for example, the one in which a surface of a carrier particle made of carbon or inorganic compound supports a noble metal fine particle is generally used. Also, because a catalytic action is exerted mainly on the surface of the noble metal, it is effective to reduce the primary particle diameter and increase the specific area ratio of the noble metal fine particle supported on the surface of the carrier particle in the catalyst having the structure described above, in order to reduce the usage of noble metals while keeping a good catalytic action.
Examples of the method for producing these fine metal particles include a high-temperature processing method called an impregnating method, a liquid phase reduction method, and a gas phase method. In recent years, a liquid phase reduction method that can simplify a production facility has been prevalent. That is, a method for using a reducing agent to reduce the metal ions to be deposited in a liquid phase reduction system and deposit the metal fine particles has been prevalent. The advantage of metal fine particles produced by the liquid phase reduction method is that the metal fine particles are not only uniform in the shape of a sphere or grain, but also are suitable especially as a fuel cell catalyst due to a sharp particle size distribution and the small primary particle diameter.
For example, Japanese Patent Application Publication No. 2003-166040 (JP-A-2003-166040) discloses the following method as a method for synthesizing monodisperse alloy fine particles having uniform particle diameter and consisting of a transition metal and noble metal. This method dissolves at least one salt or complex of a transition metal selected from Fe and Co, and at least one salt or complex of noble metals selected from Pt and Pd, into an organic solvent miscible with water or an alcohol, in the presence of an organic protective agent, and heating and refluxing it with the alcohol in an inert atmosphere, to produce a binary alloy consisting of the transition metal and the noble metal.
However, lacking in conciseness and the high cost are the problems of the method for producing metal fine particles using the liquid phase reduction method described above, because this method is a synthesizing method performed under a high-temperature reaction in the organic solvent.